In my above identified patent application there is described an improved process for drying and pelletizing sewage sludge to form pellets of small and uniform sizes. Such pellets then are suitable for use as a soil conditioner, or as a fuel. While it has been known for many years that sewage sludge could be pelletized and could be used as a fuel or soil conditioner, production of pellets in uniform sizes was not achieved prior to my invention.
In my sludge treatment process, dewatered sludge cake was mixed with a previously pelletized sludge particles and passed through a dryer. The particles of previously dried sludge assist in pelletizing the sludge cake by forming a nucleus for the formation of the pellets. The exiting materials then are separated from the of gas stream and clarified. Fines are removed and conveyed to a recycle bin. Pellets of the desired size are separated and routed to storage for shipment or sale. Oversized particles are routed to a crusher and crushed. The crushed particles are then mixed with the fines and recycled with the incoming dewatered sludge cake. In this way, the product produced for sale, is constituted only of pellets of uniform size and does not contain oversized particles or fines.
While the process requires the use of a fuel for drying the sludge cake mixture, as described in my application, the fuel could be a fossil fuel, or coal or waste wood chips or the like, or a mixture. While the fuel could present a major expense, in many municipalities, there is no alternative means to dispose of sewage sludge.
One unacceptable alternative in many municipalities is the dumping of the sludge in a nearby river. Obviously, federal regulations now prohibit such practices. Another alternative would be to dispose of the sludge in a landfill. However, in the case of many municipalities, landfill capacity is either not present or will not be in the predictable future.
In addition to sewage sludge, in many municipalities the municipal water must also be treated to "soften" it for distribution. In many municipalities the water source contains excessive concentrations of calcium and magnesium salts. Such "hard" water requires excessive quantities of soap, and leave scum which many stain fabrics. It is desirable then to treat the water to remove these calcium and magnesium salts before distribution.
A common water treatment process involves the addition of lime, or lime and soda ash, to precipitate the calcium and magnesium salts as calcium carbonate and magnesium hydroxide. This process however results in a by-product called lime sludge which also presents a disposal problem. Typically, the lime sludge is initially treated to remove the magnesium hydroxide and then is recalcined to regenerate lime from the sludge for reuse in the water treatment process. This then minimizes the disposal problem relative to water treatment.
Recalcination, is typically carried out at temperatures of 1600.degree.-1800.degree. F. and produces off gases at 300.degree.-350.degree.. This then also produces a requirement for fuel in a situation wherein alternate disposal means are not available.